Up to now, TBC systems rely on a ceramic layer of yttria-stabilized zirconia deposited either by thermal spray (for instance atmospheric plasma spray) or by a vapor deposition method (like physical vapor deposition); deposited on a MCrAlY or a PtAl bondcoat which is deposited on the base material.
Up to now, the first choice TBC material is the 6-8 wt % yttria stabilized zirconia (YSZ) composition described in U.S. Pat. No. 4,485,151. TBC prepared with 6-8 wt % yttria stabilized zirconia are after the coating process composed in the majority of a metastable tetragonal phase, which decomposes during exposure at high temperature. This decomposition process can lead to the destabilization and delamination of the TBC system.
During the past twenty years, numerous attempts have been performed at improving (TBC) systems.
Two main objectives have been targeted:                Reduce the thermal conductivity of the TBC materials, such to allow a reduction of the cooling needs        Increase the high temperature stability of the TBC materials, such to allow operating coated parts at higher surface temperature.Both of these objectives are linked to a reduction of the cooling needs, which will be correlated with an increase of gas turbine efficiency.        
During the first studies on improved TBC materials, the focus was on using zirconia-based materials and optimizing their doping such to obtain either an improved high temperature stability of the material or a reduced thermal conductivity.
In the context of the present invention of the following documents seem noteworthy:
U.S. Pat. No. 4,335,190 describes a multilayer system made of yttria stabilized zirconia with an inner layer which is around 1.5 pm thick. U.S. Pat. No. 5,840,434 discloses a multilayer zirconia coating with an outer layer having a columnar structure. EP 0 605 196 discloses a multilayer zirconia coating with 0% porosity in the inner layer and 10-20 & porosity in the outer layer. U.S. Pat. No. 6,930,066 discloses a single layer zirconia coating stabilized with more than 30 wt % Y2O3.EP1 514 953 describes a multilayer zirconia coating with an outer layer consisting of cubic YSZ. U.S. Pat. No. 6,887,595 discloses a multilayer system with an outer layer consisting of cubic zirconia stabilized with 1) Yb, Nd, Yb+La, Nd+La (5 to 49 mol %); 2) Y, Ca, Ce, Sc, Mg, In, (<4 mole %); 3) Hf (0.5 to 40 mole %) or Ta (0.5 to 10 mole %).U.S. Pat. No. 4,328,285 describes a single layer coating made of ceria stabilized zirconia. WO01/83851 describes a multilayer system resistant to environmental contaminants, with an outer layer composed of ceria-stabilized zirconia, and the outer layer being significantly thinner than the inner layer.U.S. Pat. No. 6,812,176 and U.S. Pat. No. 7,186,466 describe a single layer coating made of zirconia stabilized with multiple cluster forming doping elements, the doping elements are for the majority rare-earths. EP 1 550 642 discloses a single layer coating made of YSZ (>91 mol %)+1) Y, Ca, Ce, Sc, Mg, or In+2) La, Gd, Nd, Sm, or Dy+3) Yb or Er.EP 1 550 645 describes a single layer coating made of YSZ doped with La and Nd or doped with La and Yb. EP 1 627 862 describes a coating made of Lanthana doped zirconia stabilized with one element from the group of Y, Gd, Ca, Ce, Mg, Sc, In. U.S. Pat. No. 6,890,668 describes a single layer coating made of (Er, Nd, Sm)—SZ having a cubic fluorite structure. EP 1 588 992 describes a multilayer coating made of Hf—SZ doped with Y, Ca, Ce, Sc, Mg, In, La, Gd, Nd, Dy, Er, Yb, Eu or Pr.U.S. Pat. No. 4,913,961 describes a single layer coating made of Sc—SZ.U.S. Pat. No. 4,335,190 describes a multilayer system made of calcia-stabilized zirconia with an inner layer around 1.5 pm thick.WO0183851 (priority date 27 Apr. 2000) describes a multilayer system resistant to environmental contaminants, with an outer layer composed of calcia-stabilized zirconia, and the outer layer being significantly thinner than the inner layer.EP 1 507 022 describes a single layer coating made of YSZ doped with a pentavalent oxide which can be Ta (1-4 mol %).US2002164430 describes a single layer coating made of CaZrO3 where Ca is partially substituted with another element like Sr.EP 1 900 848 describes a multilayer coating with an outer layer made of a material having a garnet structure, the coating will reduce sand related distress.U.S. Pat. No. 6,863,999 describes a single layer coating of a rare-earth element phosphate (xenotime or monazite).JP63274751 describes a multilayer coating with an outer and inner layer system made of stabilized zirconia and the middle layer is composed of a spinel.US 2006/0078750 describes layer structures, in which, on a component, a first bond coat layer is applied and then a first ceramic layer consisting of 7YSZ. On this first ceramic layer there is provided a second ceramic layer. Among different possibilities, for the second ceramic layer multiple rare earth doped yttria stabilized zirconia is proposed. Analogous structures are disclosed in U.S. Pat. No. 6,887,595 as well as in EP 1806435.